The flow of food and social organization in the fire ant Solenopsis invicta.

In social insects, the distribution of workers within and among tasks occurs without a central authority. To determine the mechanisms regulating worker labor, the flow of food from the environment to the larva was investigated using the fire ant, Solenopsis invicta. Replicated experiments were conducted on both laboratory- and field-reared colonies from Tallahassee, Florida, U.S.A. The frequency and duration of large numbers of individual food exchanges under experimental conditions were quantified using video-technology. Treatments included worker or larval food-deprivation (marked by food dyes), body size, age, food type, food concentration, food state, nest temperature, colony size and worker:larva ratio. During each worker-larva food exchange (trophallaxis), larvae were fed a discrete increment of food (- .5 ± 0.1 nl) regardless of larval attributes and conditions. Therefore, the total volume of food ingested by larvae was determined by the rates of trophallaxis. Larvae regulated their diet by soliciting feedings from workers via a hunger cue at a rate proportional to their size and in relation to food deprivation, food type and food concentration. Nutrients were homogenized and evenly distributed among larvae, over time, per unit of larval volume. Workers displayed considerable variation in their feeding response to larval hunger which affected the time required to fill larvae but not the even distribution of food among larvae. Within naturally occurring parameters, neither temperature, colony size, nor worker:larva ratios affected the rate of worker-larva trophallaxis. Foragers assessed food on site and recruited others in relation to food type and concentration. When offered two food types simultaneously, workers moved proteinaceous solutions to larvae and sugar solutions to workers, suggesting that the behavioral response of workers was based upon crop contents. The absence of protein in worker crops rather than the presence of larvae caused workers to forage for amino acids preferentially to sucrose. Food distribution among workers was uneven and may ensure that some workers will be empty enough to forage at all times. In summary, colony nutrition is regulated by two feedback cues: worker crop content, which determines the rate at which food moves from the environment into the colony, and larval hunger which determines the rate and direction at which food moves within the colony